使用新的Linq-to-NHibernate提供程序/通过新的Linq-to-NHibernate提供程序计算Distinct -内连接- Group

Question

如何使用新的Linq到NHibernate提供程序(3.x)编写这个精确的SQL查询？

SELECT Post.Title, COUNT(DISTINCT Comment.UserId)
FROM Post
INNER JOIN Comment ON Post.Id = Comment.PostId
GROUP BY Post.Title

这里有一些SQL如果您想做一些测试，

DECLARE @Post Table(Id int identity(1,1), Title varchar(200))
DECLARE @Comment Table(Id int identity(1,1), PostId int, Comment varchar(200), UserId int)

DECLARE @PostId int

INSERT INTO @Post(Title)
VALUES ('Test')

SELECT @PostId = SCOPE_IDENTITY()

INSERT INTO @Comment(PostId, Comment, UserId)
VALUES (@PostId, 'Test Comment', 1)

INSERT INTO @Comment(PostId, Comment, UserId)
VALUES (@PostId, 'Test Comment 2', 1)

INSERT INTO @Comment(PostId, Comment, UserId)
VALUES (@PostId, 'Test Comment 3', 2)

INSERT INTO @Post(Title)
VALUES ('Test 2')

SELECT @PostId = SCOPE_IDENTITY()

INSERT INTO @Comment(PostId, Comment, UserId)
VALUES (@PostId, 'Test Comment', 1)

INSERT INTO @Comment(PostId, Comment, UserId)
VALUES (@PostId, 'Test Comment 2', 2)

INSERT INTO @Comment(PostId, Comment, UserId)
VALUES (@PostId, 'Test Comment 3', 3)


SELECT Post.Title, COUNT(DISTINCT Comment.UserId)
FROM @Post Post
INNER JOIN @Comment Comment ON Post.Id = Comment.PostId
GROUP BY Post.Title

Answer 1

我认为目前不可能完成count(distinct x)部分。

这是我最接近的：

from comment in session.Query<Comment>()
group comment by comment.Post.Title
      into g
      select new
             {
                 Title = g.Key,
                 Count = g.Select(x => x.UserId).Distinct().Count()
             };

但它产生的SQL与以下内容完全相同：

from comment in session.Query<Comment>()
group comment by comment.Post.Title
      into g
      select new
             {
                 Title = g.Key,
                 Count = g.Count()
             };

即：

SELECT Post.Title, COUNT(*)
FROM Comment
LEFT JOIN Post ON Post.Id = Comment.PostId
GROUP BY Post.Title

您应该向http://jira.nhforge.org发布一个问题。Linq提供者正在进行大量的工作，并且在不久的将来有很好的机会来支持这个构造。